Add chromium_code to tracing.gyp to build with warnings.

Also remove the defines! section which seems unnecessary (as far as I
can tell none of the code impacted includes anything from //content).

Both of these are per suggestion from thakis@.

BUG=none

Review URL: https://codereview.chromium.org/62833009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237563 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237563